This invention relates generally to an improved cross-connect system for digital telecommunication systems, and, more particularly, to an electronic digital signal cross-connect system (hereinafter EDSX) which can serve as a direct replacement for conversational manual digital signal cross-connect systems (hereinafter (DSX), and which includes built-in expanded features such as performance monitoring and test access which are not available with such conversational manual DSX.
A conversational manual DSX system is shown in FIG. 1 in conjunction with various telephone communication equipment which such manual DSX systems are customarily connected to (e.g. electronic switching systems such as 5ESS, channel banks, subscriber loops, office repeater bays, etc.). Such manual DSX systems are presently found in virtually every telephone central office in the United States. Present DSX-1 systems receive DS1 signals at the DS1 level (1.544 MBPS) and mechanically cross-connect the signals in a wired array to distribute the signals to other offices or long distance carriers. If changes have to be made in the connection, these are temporarily done by plugs and jacks, until re-wiring can be carried out. Obviously, it is desirable to develop an electronic way to achieve this distribution.
FIG. 2 shows a digital cross-connect system (DCS-3/1) which has been developed by various companies such as AT&T, NEC and Rockwell to try to enhance the operation of the conventional mechanical DSX. These systems receive signals at the DS3 level (45 MBPS), and demultiplex these to a DS1 level for processing. In processing direct DS1 inputs, these systems convert the signals to a binary level (from the bipolar 3-level DS1 signals having +, 0 and - levels) thereby losing the bipolar information. These systems have significantly more delay than the mechanical DSX system (which has almost no delay because of the fact that it is a simple wired connecting arrangement). If a number of these DCS-3/1 systems are used together, the delay becomes intolerable, and an echo canceller must be used (at significant added cost). It should be noted that basically these systems operate as time and space switching arrays.
Although such DCS systems are capable of handling very large volumes, they do not solve the problem of providing a direct electronic substitute for the conversational manual DSX. For one thing, as noted above, the DCS systems employ time and space switch arrays which cause significant delays in the signal processing. The manual DSX, on the other hand, has virtually no delays at all in cross-connecting various signal lines. Accordingly, the DCS systems cannot serve as direct transparent replacements for the manual DSX systems because of the greater delay which they bring about.
In addition to the delay found in DCS systems, they also suffer from another problem which prevents them from satisfactorily serving as a direct transparent replacement for manual DSX. Specifically, DCS systems currently in use are not transparent to bipolar violations. An explanation of this shortcoming is provided below.
As referred to above, DS1 signals are conventionally formed as bipolar AMI (alternate mark inversion) 3-level signals which are pseudoternary signals, conveying binary digits, in which successive "ones" (marks, pulses) are of alternating, positive (+) and negative (-) polarity, equal in amplitude, and in which a "zero" (space, no pulse) is of zero amplitude. At times, deliberate violations are made of the AMI code, for example to test the overall communication system to ensure proper function. These are customarily referred to as bipolar violations and occur when a one (mark, pulse) has the same polarity as its predecessor. A conventional manual DSX system is transparent to such bipolar violations, so that any intentional bipolar violation will be the same at the output of the manual DSX as it is at the input thereof. However, because the DCS system breaks down the bipolar 3-level signals to a simple 2 level digital signal for faster and simpler proceeding, such bipolar violations are lost. In other words, a deliberate bipolar violation at the input of a DCS system will not show up as a bipolar violation at the output thereof. Accordingly, DCS system are incapable of bipolar violation transparency which is available in manual DSX systems.
Although manual DSX systems have the advantage of virtually no delay and bipolar violation transparency, they do, of source, suffer from a number of serious drawbacks. As noted above, they require re-wiring whenever a cross-connection change is to be made. This, of source, is extremely cumbersome. In addition to this, manual DSX cannot be electronically controlled from the outside by either craft interface terminals (CIT) or external operating systems (OS). Further, there is no provision in conversational manual DSX for built-in performance monitoring. Accordingly, it would be highly desirable to provide an EDSX which is capable of electronically changing the cross-connections, and which is also capable of the above-noted functions which the manual DSX is incapable of, while still maintaining the desirable features of virtually no delay and bipolar violation transparency.